My six y/o dell laptop is powered by intel pentium M processor 1.6ghz 591mhz, 512mb ram, may I know with this spec, should I consider Ubuntu or kubuntu or any other os?

Main usage just for watching movie (rmvb mostly and mp4), surfing net.

Now using window xp and my rmvb Movie sound lagging for few seconds for unknown reason,try other player also same ...so thinking may give it a try on other os